xarray: make xa_alloc_cyclic() return 0 on all success cases

Change xa_alloc_cyclic() to return 0 even on wrap-around. Do the same for
xa_alloc_cyclic_irq() and xa_alloc_cyclic_bh().

This will prevent any future bug of treating return of 1 as an error:
int ret = xa_alloc_cyclic(...)
if (ret) // currently mishandles ret==1
goto failure;

If there will be someone interested in when wrap-around occurs, there is
still __xa_alloc_cyclic() that behaves as before. For now there is no
such user.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Przemek Kitszel <[e-mail]>
Suggested-by: Matthew Wilcox <[e-mail]>
Link: https://lore.kernel.org/netdev/[e-mail]
Cc: Andriy Shevchenko <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Michal Swiatkowski <[e-mail]>
Cc: Przemek Kitszel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/xarray.h

···
  * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
  * in xa_init_flags().
  *
+ * Note that callers interested in whether wrapping has occurred should
+ * use __xa_alloc_cyclic() instead.
+ *
  * Context: Any context.  Takes and releases the xa_lock.  May sleep if
  * the @gfp flags permit.
- * Return: 0 if the allocation succeeded without wrapping.  1 if the
- * allocation succeeded after wrapping, -ENOMEM if memory could not be
+ * Return: 0 if the allocation succeeded, -ENOMEM if memory could not be
  * allocated or -EBUSY if there are no free entries in @limit.
  */
 static inline int xa_alloc_cyclic(struct xarray *xa, u32 *id, void *entry,
···
 	err = __xa_alloc_cyclic(xa, id, entry, limit, next, gfp);
 	xa_unlock(xa);
 
-	return err;
+	return err < 0 ? err : 0;
 }
 
 /**
···
  * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
  * in xa_init_flags().
  *
+ * Note that callers interested in whether wrapping has occurred should
+ * use __xa_alloc_cyclic() instead.
+ *
  * Context: Any context.  Takes and releases the xa_lock while
  * disabling softirqs.  May sleep if the @gfp flags permit.
- * Return: 0 if the allocation succeeded without wrapping.  1 if the
- * allocation succeeded after wrapping, -ENOMEM if memory could not be
+ * Return: 0 if the allocation succeeded, -ENOMEM if memory could not be
  * allocated or -EBUSY if there are no free entries in @limit.
  */
 static inline int xa_alloc_cyclic_bh(struct xarray *xa, u32 *id, void *entry,
···
 	err = __xa_alloc_cyclic(xa, id, entry, limit, next, gfp);
 	xa_unlock_bh(xa);
 
-	return err;
+	return err < 0 ? err : 0;
 }
 
 /**
···
  * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
  * in xa_init_flags().
  *
+ * Note that callers interested in whether wrapping has occurred should
+ * use __xa_alloc_cyclic() instead.
+ *
  * Context: Process context.  Takes and releases the xa_lock while
  * disabling interrupts.  May sleep if the @gfp flags permit.
- * Return: 0 if the allocation succeeded without wrapping.  1 if the
- * allocation succeeded after wrapping, -ENOMEM if memory could not be
+ * Return: 0 if the allocation succeeded, -ENOMEM if memory could not be
  * allocated or -EBUSY if there are no free entries in @limit.
  */
 static inline int xa_alloc_cyclic_irq(struct xarray *xa, u32 *id, void *entry,
···
 	err = __xa_alloc_cyclic(xa, id, entry, limit, next, gfp);
 	xa_unlock_irq(xa);
 
-	return err;
+	return err < 0 ? err : 0;
 }
 
 /**

lib/test_xarray.c

···
 	unsigned int i, id;
 	unsigned long index;
 	void *entry;
+	int ret;
 
 	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(1), limit,
 				&next, GFP_KERNEL) != 0);
···
 		else
 			entry = xa_mk_index(i - 0x3fff);
 		XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, entry, limit,
-					&next, GFP_KERNEL) != (id == 1));
+					&next, GFP_KERNEL) != 0);
 		XA_BUG_ON(xa, xa_mk_index(id) != entry);
 	}
 
···
 				xa_limit_32b, &next, GFP_KERNEL) != 0);
 	XA_BUG_ON(xa, id != UINT_MAX);
 	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(base),
-				xa_limit_32b, &next, GFP_KERNEL) != 1);
+				xa_limit_32b, &next, GFP_KERNEL) != 0);
 	XA_BUG_ON(xa, id != base);
 	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(base + 1),
 				xa_limit_32b, &next, GFP_KERNEL) != 0);
···
 
 	xa_for_each(xa, index, entry)
 		xa_erase_index(xa, index);
+	XA_BUG_ON(xa, !xa_empty(xa));
 
+	/* check wrap-around return of __xa_alloc_cyclic() */
+	next = UINT_MAX;
+	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(UINT_MAX),
+				      xa_limit_32b, &next, GFP_KERNEL) != 0);
+	xa_lock(xa);
+	ret = __xa_alloc_cyclic(xa, &id, xa_mk_index(base), xa_limit_32b,
+				&next, GFP_KERNEL);
+	xa_unlock(xa);
+	XA_BUG_ON(xa, ret != 1);
+	xa_for_each(xa, index, entry)
+		xa_erase_index(xa, index);
 	XA_BUG_ON(xa, !xa_empty(xa));
 }